EPISODIC DEFORMATION AND VARIATIONS IN FAULT SLIP RATES DURING THE LATE QUATERNARY IN THE WESTERN TRANSVERSE RANGES, CALIFORNIA

At the western end of the Transverse Ranges west to northwest striking reverse faults and folds deform Quaternary sediments within, and at the edges of, the Santa Maria Basin. We documented the amount and timing of deformation across these structures by measuring displacement of fluvial terrace deposits across faults at the surface, reconstructing stratigraphic horizons to infer displacement along blind faults beneath the larger fold structures, and by dating these deposits with Infrared Stimulated Luminescence of feldspar. The regionally extensive Orcutt Formation is a fluvial deposit developed on a low-relief surface and is an excellent marker for late Quaternary deformation in the area. Luminescence ages from 8 different sites across the region show that the Orcutt Formation was deposited between 80 ka and 110 ka. Reconstruction of the basal contact of this unit reveals a background uplift rate of about 1mm/yr since 100 ka in most areas, with higher uplift rates of 2 to 4 mm/yr across the Purisima, Casmalia hills, and Santa Rita Hills, which are fold structures above a blind fault system. Restoration of cross-sections across the area results in slip rates of 6.5 to 9 mm/yr on the low-angle faults underlying the Purisima and Casmalia hills. The Santa Rita Hills grew as a result of slip on the blind Santa Ynez River Fault after 80 ka, but slowed in the latest Pleistocene and there is no deformation of Holocene terraces. Similarly, the Baseline fault to the east became active after Orcutt deposition and offsets 55 ka and 30 ka fluvial terraces, but does not offset Holocene terraces. These results indicate that many of the major structures in this part of the western Transverse Ranges were active between 85 ka and 30 Ka after a period of tectonic quiescence and topographic planation, but that much of the deformation subsequently decreased and has not continued into the Holocene. Large angular unconformities between the base of the Orcutt formation and the underlying Pleistocene Paso Robles formation, another regionally extensive fluvial plain deposit, suggest that another cycle of planation and deformation may have occurred earlier in the Pleistocene. The emerging picture is that deformation and associated uplift is not constant in this part of the western Transverse Ranges, but rather episodic over the past 100 Ka or more.